Sure Electronics manufacture a cheap Microchip MRF24J40 2.4GHz 802.15.4 Radio board with a PIC24FJ32GA002 Microcontroller (GP-GC500).

• Sure Electronics Explorer 2.4GHz Wireless UART Development Kit Schematics (GP-GC500)
• Simple driver code for Microchip MRF24J40 radio - Nerd Fever
• IEEE 802.15.4 Specification for for Low-Rate Wireless Personal Area Networks (WPANs)